1. Field of the Invention
This invention relates generally to tank gauging systems of the type incorporating a free-float. More particularly, this invention relates to an electrostatically safe float for use in such systems.
2. Description of the Prior Art
One technique prevalent today in the measurement of liquid level within a storage tank includes a guide to be vertically supported within the tank. A donut-shaped float is positioned about the guide and is supported by the liquid surface. As the float rides with the liquid level, its position is sensed by suitable means to indicate the level of the liquid within the tank.
This type of system is generally acceptable in most applications, but presents a potential electrostatic hazard when applied in tanks containing flammable liquids. In storage tanks which are partially open to the surrounding atmosphere, such as cone-roof tanks, the electrostatic hazard is greatly increased.
It is well known in the art that during the process of pumping a liquid into a storage tank, the liquid will become electrostatically charged. The rate of accumulation of this charge is related to the flow rate and conductivity of the liquid. If the rate of accumulation of charge is greater than the rate at which it is neutralized, a net charge will be present on the bulk of the liquid, and consequently on the float. For example, at flow rate in excess of one meter/second, liquids having conductivities less than 50 picosiemens may develop a net charge of 30-40 K volts during the filling operation.
This net charge will be totally neutralized within a predetermined time period, called the Relaxation Time. The Relaxation Time may range from seconds for crude oil, to many minutes for gasoline or jet fuel.
During this time period, if the float comes sufficiently close to or in contact with, the vertically supported guide an electrostatic spark of sufficient energy may be developed in the tank's vapor space to cause an ignition of the flammable vapors.
One prior art solution calls for the float to be continuously tied to electrical ground by a bonding wire. In this manner, the two elements (i.e., the float and the wire) are maintained at the same electrical potential. The drawback of this approach is the maintenance of a reliable bond while allowing the float to ride freely with the liquid surface.
Another prior art solution calls for the use of a radioactive source contained within the float. The source ionizes the vapor space between the float and the guide, causing it to become conductive. This provides a low impedance discharge path between the float and the guide, such that static electric charge generated during the filling operation is neutralized before a sufficient net accumulation of charge to cause a spark is generated. The obvious drawback with this approach is customer acceptance of the radioactive source.